Hello, I'm building my first greenhouse and need some advice. First I live in Iowa in climate zone 5 and this will be a four season greenhouse. The structure will be 20' X 12' and will sit on a heavily insulated 44" kneewall that will be below grade on three sides. The greenhouse will have lowE double pane glass on the sides and will have 5 wall polycarbonate on the roof. The greenhouse is being built in Canada and will arrive one month from now. The question I have is should I put insulation below the concrete slab or should I use the ground under the slab as a heat sink? I'm just curious what the experts here would recommend. I've got a 45K BTU furnace so it should keep it warm with or without floor insulation.
Thanks in advance,
Keith

I'm looking to build a greenhouse that will also be 4 seasons. Do you mind sharing where you got your greenhouse at?

Sorry I don't have an answer on the insulated foundation part.

I applaud your plan so far; for both summer and winter the high R glazing is the simplest solution (compared to higher insulating panels that must be moved and managed).

I have experience designing and building a home using sub-slab rock and earth as heat storage, and subsequent building experience with a home with insulated slab.

In my opinion the earth is overrated as a storage medium. The earth is likely to not reach seasonal equilibrium until a dept of 12 feet or more in your area. I had sensors buried, and even when the outdoor temps were above any upwards loss, I could watch heat being sucked out of my storage area below the rocks even when there was no loss above. I concluded that the earth is an endless heatsink except on much larger scale than I could build. In your area, I'd guess 55F down there at maybe 14 or 15 feet. You need to insulated both bottom and sides for it to work as a storage mass.
Earth is lossy and of piss poor thermal value compared to water. I now heat my home with just 800 gallons of insulated, solar heated water with a backup propane heater for the rare extended dark days.

If you plan to supplement heating, by thermal water storage or other means, I recommend you insulate the floor well, especially at the edges. Huge losses there, and the relatively poor performance of 30 inch wide soap bubble insulated high tunnels proves it, since they have outstanding glazing insulation value at night. The earth outside has a broad thermally conducive path to the inside, especially when the earth is damp or wet (and frozen). We live above ground so forget what is below, and heat does not.
If you set a hot frying pan on your hand, you will see that heat conduction downward or sideways works just fine, heat does not only rise by convection.

If you want to directly plant in the soil, the at the very least, get a trencher and bury 48 inch wide extruded polystyrene panels around the perimeter. That's still going to be a big loss downwards and outwards but more so in late winter as opposed to early.

Best Wishes
Bruce

mtngoat, I'm getting my greenhouse from the Wisconsin greenhouse company but I believe they're just a distributor for a greenhouse company in British Colombia Canada. My sunken kneewall will be made with Insulated Concrete Forms (ICF) so the walls will be nearly 12" thick with 3" of foam on sides with a core of 6" concrete (that's before I put tile on the inside and brick on the outside). Three sides of the kneewall will be below grade but my soil is 100% wet clay which Bruce pointed out will be thermally conductive. From what I'm hearing it sounds like I should be some foam insulation under the slab.
Thanks,
Keith

Congratulations on building what sounds like a really nice GH, Keith!

I'm curious about the use of the low-E glass -- we have a sunroom with low-E windows and it actually blocks too much of the light for even most seedlings to develop normally. (Of course, that would be very early in the year and maybe the problem is just that we aren't getting that much daylight at that time?)

This conversation would be more relavant if we knew were Bruce, Keith and MntGoat lived. Maybe someplace cold? The greenhouse might be a little overbuilt for Florida.

Don't know about the others, but in his first post Keith said he's in Iowa, zone 5.

Weedwacker, you're right LowE glass has 70% light transmission while double pane glass is close to 90%, I only went with lowE glass because we can hit -28F here. I think the 5 wall plastic is around 85%.
Keith

DaisyI said:This conversation would be more relavant if we knew were Bruce, Keith and MntGoat lived. Maybe someplace cold? The greenhouse might be a little overbuilt for Florida.

I don't want to intrude too much on this post since it wasn't originally mine but I'm in Kansas. Just wanting to learn more before I spend money on a greenhouse since it looks like it'll be pretty pricey.

mntgoat said:

I don't want to intrude too much on this post since it wasn't originally mine but I'm in Kansas. Just wanting to learn more before I spend money on a greenhouse since it looks like it'll be pretty pricey.

You are never intruding unless you ask a new question and confuse us all. I was trying to figure out how cold is cold. Zone 4, zone 5 and one Kansas.

My floor is a slab also. I don't think insulation without heat will help heat your greenhouse as the slab is inside, away from the sun and unless you dig down about a mile, earth heat won't help. My greenhouse slab has hot water piped under it. Its nice and warm in winter but not because of the sun. Once you fill up a greenhouse, there isn't any floor for the sun to shine on anyway.

A 4-season greenhouse can mean different things; if you want to grow tropicals during a northern winter, then yes, definitely going to be pricey. But if you want to emulate Eliot Coleman's "Four Season Harvest," then not so much.

Let me chime in here, check with City or County laws before you build! Here you need a permit due to a company came in and poured cement floors in 6 greenhouses. This caused us all to go through the Country process.

That is true! I had to stick to a 15 ft dome greenhouse as it was counted as a shed. Anything bigger required permits and heavy taxation.

Yes, lots of permits here. I've gotten permits for electric, gas, water, and foundation so far and will need another one when the greenhouse is completed. I ran the water line 5'-6' deep to prevent freezing. Getting utilities to the site was no easy task. I'll post pics when the structure goes up.
Keith

It sounds great, Keith! Good luck, and we'll look forward to your pics.

Didn't want a concrete slab and the drainage problems. For a 12 x 12 ft. greenhouse, year-round in a savage climate (-40°F winter nights) I set an insulated foundation about 3 ft. below grade. Laid 2-inch foamboard over the space and filled in with a foot of gravel, then coarse sand. Laid a maze of PEX tubing for radiant floor heat. There's a flat-plate collector running a glycol loop through a big heat sink water tank in the floor, with DC pumps that run off solar panels.


Finished with framed sand beds and dark pavers.

I decided to go with 2" EPS foam under 8" of concrete for the slab. I wrote a radiant heat company and they said there was no way I could heat a 250 sq foot greenhouse with radiant hot water floor heat so I went with a 45K BTU furnace.
Keith

Mine is heated with radiant hot water floor heat but I had to add a 4 more hoses to the wall to accomplish a minimum 50 - 55 degree temp. I use a small utility heater for emergency but I don't need it often. Our minimum temperatures are about 0 at the lowest.

Mounted a surplus flat-plate collector at a steep angle (for winter sun) and attached it to a loop of PEX tubing filled with 50% glycol to prevent freezing.


The small PV panel drives a DC pump that circulates the glycol solution through a 400 gal. water tank under the floor— the heatsink.


With good sun, water temps in the heatsink can get above 120°F. An AC pump circulates the hot water through a maze of PEX tubing in the sand floor.

I also mounted four black tanks on a south-facing wall, as passive heatsinks. They're also the source for watering plants, so there's no conventional plumbing.

Took apart a quartz radiant patio heater and mounted the elements on the ceiling, on a thermostat, to warm the foliage directly.


For severe cold, I have a small propane radiant heater attached to an outside tank. It gets used only a few times each winter. I've lost a few plants to cold air leaks at the corner intake vents, but in 12 years it's never frozen inside.

I'm having major delays on my foundation but the greenhouse shipment is also delayed. Fortunately, it's been a really warm fall here in Iowa. Here's a picture of my foundation. When it's finished it will be backfilled so only the brick will be exposed. I went with a 12" thick insulated slab. The walls were made with insulated concrete forms. The slab pour took over 10 yards of concrete so that's a 40000lbs slab that should do a great job absorbing and radiating heat.
Keith